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Behavior of Phosphoric Acid in the vSoil

A THESIS.

SUBMITTED TO THE UNIVERSITY FACULTY OF CORNELIy

UNIVERSITY FOR THE DEGREE DOCTOR

OF PHIIvOSPHY

BY

JAMES ADRIAN BIZZELL

1903

PRESS OF

THE ITHACA JOURNAL

ITHACA, N. Y.

The author takes this occasion to acknowledge the kindness
of Professors G. C. Caldwell, L. M. Dennis and E. M. Chamot,
of the Department of Chemistry, under whose guidance the fol-
lowing work was carried out.

CONTENTS

Historical 5

Summary 20

Experimental

I. Methods of Determining Phosphoric Acid 22

II. Relation between Phosphoric Acid and Compounds

of Iron and Calcium 23

(a) action of lime on ferric phosphate 24

(b) action of lime on the phosphate formed

by fixation 27

(c) action of lime on soils 31

(d) fixation of phosphoric acid in the

presence of compounds of iron and

calcium 34

III. Behavior of Phosphoric Acid towards Humus 35

(a) absorption

(b) solution 40

IV. Absorption of Phosphoric Acid by Zeolites 41

V. Action of Ferrous Sulphate 44

Conclusions 46

239388

HISTORICAL

The fact that soils are capable of absorbing certain substances"
from solution has been known for nearly a century. There has
been much discussion as to whom the credit for this discovery is-
due. The first authentic record appears to have been made by
Gazzeri in 1819. He says : "If extract of dung strongly colored
and containing nutritive matter is added to a clayey soil, the liquid:
is rapidly decolorized. The soil takes hold of the substances in
solution, and forms with them compounds which are insoluble,,
but which are decomposed by the absorbing action of plants."

Bronner in his treatise on "Grape Culture in South Ger-
many" published in 1836, made similar statements regarding the
action of sand and garden soil, and HuxtableMn 1848, apparently
ignorant of previous experiments, repeated the work of Gazzeri..
A little later the work of these observers was extended somewhat
by Thomson^ of England.

In 1850 Way^ made a systematic investigation of the absorb-
ing power of soils. His efforts were directed almost exclusively
toward ascertaining the cause and nature of the absorption of
bases. He published only one experiment on the absorption of
phosphoric acid. When a solution containing calcium acid phos-
phate was filtered through a soil, no trace of phosphoric acid was
found in the filtrate.

The next work bearing on the subject was that of Wicke^
who found that pure marble kept in contact with a solution of
superphosphate evolved carbon dioxide, and the greater part of
the phosphoric acid was precipitated.

Thenard^ was the first to make any systematic study of the
absorption of phosphoric acid by soils. He succeeded in decol-
orizing the liquid from barnyard manure by the use of iron and
aluminum hydroxides, and calcium carbonate. He concluded

1 Jour. Roy. Agr. See, England.

2 Ibid., 1850, II, 68.

3 Ibid., 1850, 11,313.

4 Ann. Chem. Phar., 1856, 99, 97.

5 Compt. rend., 44, 819.

3 •

that the active constituents of manure are removed in the soil by
these compounds, and that the new compounds are decomposed
slowly for the use of the plant. In a subsequent investigation^
he always found phosphoric acid combined with aluminum and
iron, and never with calcium and magnesium. He added solu-
tions of calcium phosphate to the pure hydroxides of aluminum
and iron, and to soil, and found after a few days that the filtered
water contained calcium but no trace of phosphoric acid.

Deherain"'' examined a large number of soils. Some of these
contained phosphoric acid insoluble in water charged with carbon
dioxide, while others gave up phosphoric acid to this solvent.
Soils containing compounds insoluble in carbon dioxide water
gave up phosphoric acid to water containing calcium carbonate
ammonium carbonate. A solution of calcium carbonate in water
charged with carbon dioxide acted on iron phosphate producing
calcium phosphate.

The role of phosphoric acid in the soil was taken up by
Iviebig^ in connection with his extended investigations on soil
absorption. He determined the solvent effect of sodium chloride,
sodium nitrate, and ammonium sulphate on phosphates of calcium
and magnesium. In all cases appreciable quantities of the phos-
phates were dissolved. When the solutions thus made were
added to soil the phosphoric acid was absorbed. Such changes
were thought to have a favorable influence in bringing about the
distribution of phosphoric acid in the soil.

Knop* confirmed the observations of Thenard regarding the
role of iron and aluminum hydroxides in soil absorption. He
showed that phosphoric acid was fixed in greater amount and
more rapidly when these compounds were added to soils, and
that soils containing large quantities had greater reverting prop-
erties than those containing small amounts. Data were not
accessible.

Peters^ endeavored to ascertain the state in which phosphoric
acid is held in soils. He manured a soil with bone dust and

1 Compt. rend., 46, 212.

2 Ibid., 47, 47, 988.

3 Am. Chem. Phar., 106, 185.

4 Jahresbericht, 1865, 804.

5 Ann. Landw., 1867, 31.

treated the mixture with different solvents. From his results, lie
-concluded that phosphoric acid in soils is almost entirely com-
bined with iron and aluminum, and that from a solution of cal-
•cium phosphate in carbon dioxide water, phosphoric acid is re-
moved by soils only when then the latter contain hydroxides of
iron and aluminum. Soils deprived of these compounds by treat-
ment with acids are apparently indifferent to the phosphate
solution.

In studying the fixing power of soils for phosphoric acid
Voelcker^ placed weighed quanties of six different kinds of loams
in bottles, and added known quantities of superphosphate solu-
tion. He noted from time to time the amount of phosphoric acid
still remaining in solution. The hydroxides present in the soils
varied from zero to 17.38 per cent, and the carbonate of calcium
from 0.15 per cent to 67.5 per cent. The phosphoric acid was
fixed with especial ease by those soils which contained a good
store of calcareous matter. The power of the clay soils to render
the soluble phosphoric acid insoluble was far less than that of
chalky soils.

Warrington^ determined the effect of several substances on
the solubility of freshly precipitated tri-calcium phosphate. He
found that one per cent solutions of ammonium chloride increavSed
the solubility in pure water, but that its addition to water sat-
urated with carbon dioxide produced practically no effect. He
observed also that when an excess of calcium carbonate was pres-
ent, the amount of tri-calciura phosphate dissolved by carbon
dioxide water was excessively small. The water became satu-
rated with calcium carbonate, while only trace of the phosphate
•entered into solution. Even a very small amount of calcium
carbonate was capable of producing this effect.

Two years later^ the same investigator carried out an elab-
orate experiment to show that the absorptive action of soil towards
phosphoric acid is due to the formation of insoluble phosphates,
by combination with hydroxides of iron and aluminum. Soils
freed from lime were treated with solutions of calcium phosphate

1 Jour. Roy. Agr. Soc, 1863, 24, 46.

2 Jour. Chem. Soc, 1866, 19, 296.

3 Ibid., 1868, 21, I.

8

in carbon dioxide water. Almost all the phosphoric acid was re-
tained by the soil, while the greater part of the calcium went into
solution. The same results were obtained when hydroxides of
iron and aluminum were used instead of soil.

Beyer and Biedermann^ worked on the absorption of phos-
phoric acid by soils. Using a solution of sodium phosphate they
found no definite relation between the amount absorbed and the
quantities of iron, aluminum, and calcium contained in the soil,
though there was increased absorption with increased quantities
of these constituents. They determined the solvent action of
solutions of calcium sulphate, sodium chloride, sodium nitrate,
potassium chloride, and ammonium sulphate on the compound
formed by absorption. No appreciable effect was observed.

Trochot^ in his observations on volcanic soils noticed that
fair amounts of lime and phosphoric acid were present, and that
the fertility of these soils was very high in spite of their shallow-
ness. The conclusion was drawn that the lime rendered the
insoluble phosphates available.

The fact that the phosphoric acid of superphosphate grad-
ually becomes insoluble in ammonium citrate solution was
observed by Millott^ and Joulie*. Millot found the change
to be due to the presence of large quantities of iron and alumium
oxides, while the results of Joulie seem to show that when an
insufficient quantity of sulphuric acid to completely decompose
the phosphate rock is used, (a condition which obtains in actual
practice) the change is due to the formation of tri-calcium phos-
phate.

These experiments were extended somewhat by Albert and
Vollbrecht^ They applied superphosphate to some soils con-
taining large amounts of lime, and to others containing small
amounts. With soils containing small quantities of lime, 6.5
per cent of the phosphoric acid of the superphosphate had become
insoluble in ammonium citrate after eight days, while w4th those
rich in lime, 15.4 per cent of the phosphoric acid had become

1 Chem. Centr., 1869, 945.

2 Compt. rend., 81, 1027.

3 Bied. Centr., 1875, 89—1879, 408.

4 Ann. Chim. Phys., Series 5, 1879, 244.

5 Bied. Centr., 1880, 84.

insoluble. These results were construed as proving the rever-
sion of phosphoric acid by soils to insoluble phosphate of lime.
In another experiment precipitated phosphates of iron and
aluminum were added to soils with the result that none of the
phosphoric acid became insoluble in ammonium citrate during
eight days.

Petermann^ attempted to determine the crop producing
power of these various forms of phosphoric acid. The "citrate
soluble" was in many cases superior to the "water soluble,"
though the results were not altogether satisfactory. The low
value of the water soluble was due apparently to leaching, while
the citrate soluble, being insoluble in water, was retained for the
use of the plant.

Solubility determinations of the compounds supposed to
exist in superphosphate were made by Albert and Wagner^ Di-
calcium phosphate was found to be soluble in water charged with
carbon dioxide. Sulphates and chlorides appeared to have very
little solvent action, while nitrates and carbonates were more
effective. The solubility of precipitated phosphates of iron and
aluminum in carbon dioxide water was a great deal less than
that of precipitated calcium phosphate. Drying these salts les-
sened their solubility. Regarding the absorption of phosphoric
acid by soils they found a solution of the phosphates in carbon
dioxide water similar in its action to a solution of "soluble phos-
phoric acid. ' ' Absorption was most complete in clays poor in
lime, less complete in calcareous clays and organic soils, and
very small in sands poor in lime. An increase of humus in the
soils rich in lime and clay was always accompanied by an increase
in absorptive power.

In contradiction to the work just quoted, Fiedler^ states that
solutions of nitrates dissolve less phosphoric acid from soils than
pure water. His results showed absorption to be favored silghtly
by sodium nitrate. A solution of sodium nitrate did not affect
the solubility of tri-calcium phosphate but it acted on di-calcium
phosphate and phosphates of iron and aluminum.

1 Bied. Centr., 1880, 87.

2 Ibid., 1880, 640.

3 I^andw. Versuchs-Stat., 26, 135.

lO

Kostitcheff^ working with pure compounds, obtained a de-
composition of calcium carbonate in the presence of water and
phosphates of iron and aluminum. Carbon dioxide was evolved
and calcium phosphate was formed. The change took place even
when there was present a large excess of iron with respect to the
phosphoric acid.

Tuxen^ investigated two soils — a sand and a clay — in order
to ascertain the effect of chlorides of sodium and potassium, and
sodium nitrate on the absorption of plant food. Phosphoric acid
was readily taken up in the presence of the alkalies. Potassium
chloride was the most effective, causing an increase in absorption
of from 25 per cent to 40 per cent. Solutions of these salts were
found to have little effect on the solubility of soil phosphoric acid.

Krocker and Grahl^ carried on manuring experiments with
phosphates of various kinds. Oats and beets seemed to be little
benefited by the soluble phosphates when used alone. When used
in conjunction with ammonium sulphate there was decided in-
crease in the crop, the greatest yield being obtained by the use of
bone meal.

Similar experiments were made by Hoffmeister*. The vari-
ous phosphates were applied as a top dressing. The results are
interesting. In a sandy soil poor in lime the soluble phosphoric
acid did not descend more than ten inches. Di-calcium phos-
phate remained unaltered during the experiment, and the mono-
calcium phosphate was not converted into the tri-basic form.

Fleischer and Kissling^ studied the influence exerted by cer-
tain salts on the solubility of the phosphates present in peaty
soils. Among the salts used were the sulphate, chloride, and
carbonate of potassium,, the sulphate and chloride of calcium,
sodium nitrate, ammonium sulphate, and kainite. Potassium
Sulphate was very effective, while sodium nitrate and kainite
were far less so.

Gladding^ claims to have been the first to prove by direct

1 Bull. Soc. Chim., (2) 1880, 34, 341.

2 r,andw. Versuchs Stat. , 27, 107.

3 Bied. Centr., 1882, 1154,

4 Ibid., 1881, 813.

5 Ibid., 1883, 153.

6 Chem. News, 50, 16, 27.

II

laboratory experiment, that the greater part of the phosphoric
acid reverted in soils is in the form of iron and aluminum phos-
phates. On adding a solution of superphosphate to three repre-
sentative soils he found after several days that all the phosphoric
acid was soluble in ammonium citrate at a temperature of 65
degrees C. "This" the author states "shows that most of the
phosphoric acid was present in the form of iron and aluminum
phosphates, since tri-calcium phosphate is much less readily at-
tacked by this reagent than these phosphates." The author
criticizes results obtained by Albert and Vollbrecht\ from which
these investigators concluded that the phosphoric acid not dis-
solved by ammonium citrate at 40 degrees C had become insoluble
tri-calcium phosphate. Unfortunately Gladding did not give the
composition of the soils used in his experiments.

The composition and action of superphosphate have been
studied from many points of view. Weilandt^ found it to be very
completely absorbed by marl. The action was rapid. Marge-
stein^ showed its value to be increased by mixing with wood
ashes. He grew potatoes, mustard, barley, and maize on a
diluvial sandy soil. Quantities of ashes up to 25 per cent were
beneficial. Analyses of soils UvSed were not given.

From his results Joffre* referred reversion to the formation
of phosphate of iron. He determined its coefficient of solubility
in water, in water charged with carbon dioxide, and in solutions
of diiFerent salts. In all cases it was less sensitive to these
substances than tri-calcium phosphate. Vegetation tests showed
it to be less valuable.

Thompson^ determined the absorptive power of sand, as well
as that of the same sand containing known quantities of ortho-
clase, calcium carbonate, iron and aluminum hydroxides, of
mixtures of calcium carbonate and orthoclase, and of calcium
carbonate and hydroxides of iron and aluminum. He also
noticed the effect of sodium chloride and potassium nitrate on
the process of absorption. The original publication was not

1 Bied. Centr. 1880, 87.

2 I^andw. Versuchs-Stat., 34, 207.

3 Bied. Centr., 1888, 225.

4 Bull. Soc. Chim., 47, 312.

5 Inaug. Dissertation, Dorpat, 1890.

12

-accessible but abstracts give the following conclusions. ' 'Sand
offers no resistance to the extraction of the phosphoric of super-
phosphate by water. Addition of orthoclase produces no effect.
Calcium carbonate combines quickl}^ with soluble phosphoric
acid. Iron and aluminum hydroxides are also active in retain-
ing phosphoric acid. The compounds formed with iron and
aluminum are more stable towards salt solutions than tri-calcium
phosphate. Sodium chloride solutions (i per cent and 2 per
cent) dissolve less phosphoric acid from superphosphate than
pure water. In the presence of calcium carbonate and hydroxi-
des of iron and alumium the salt solutions dissolve more than
pure water. ' '

The action of carbon dioxide on tri-calcium phosphate alone
and in the presence of ferric hydroxide was studied by Geogievice\
Tri-calcium phosphate suspended in water was decomposed by
carbon dioxide, but the reaction was far from being complete.
When ferric hydroxide was also present decomposition took place,
but the phosphoric acid immediately combined with the iron.
Under some conditions the whole of the phosphoric acid was with-
drawn from the calcium salt. These results confirm the observa-
tions of many previous workers, namely, that all of the phosphoric
acid in calcium phosphate when applied to the soil finally becomes
converted to phosphate of iron.

Some interesting facts regarding the assimilation of phos-
phoric acid by crops were obtained by Deherain^ on the experi-
mental plots at Grignon. He noticed that the sum of the phos-
phoric acid in drainage and crops in ten years, was not equal
to the phosphoric acid not extracted by acetic acid. He con-
cluded that the calcium phosphate was changed to iron and
aluminum phosphates after a time. A soil was mixed with tri-
calcium phosphate and placed in a seltzogene. After a few days
no phosphoric acid was dissolved by water charged with carbon
•dioxide, although tri-calcium phosphate is soluble in this mix-
ture. He does not state whether the soil contained lime. Ferric
and aluminum phosphates were assimilated by oats but not by
wheat.

1 Monatsh., 1891, 12,566.

2 Ann. Agron., 17, 445.

13

Kellner^ made some observations on the action of lime as-^-
manure on paddy fields. Complete infertility had resulted in
several places. He attributed part of this to loss by leaching.
Among other things the action of phosphates in two soils was in-
vestigated. He mixed in from 0.25 percent to 5 per cent of
quicklime, and after two weeks added a solution of potassium tri-
hydrogen phosphate. Lime apparently caused an increase in the
soluble phosphoric acid. The maximum effect was obtained with
I per cent to 2.5 lime. After two months still more phosphoric
acid had become soluble.

Gerlach"'^ repeated Thompson's work^on the absorption of
phosphoric acid by soils. The results here recorded show that
clay, peat, and sand which have been extracted with hydrochloric
acid have no power of absorbing free phosphoric acid, sodium
phosphate, and superphosphate. Calcium and magnesium car-
bonates absorb phosphoric acid, while iron and aluminum oxides
do so very completely. In the first case the compounds formed
are comparatively soluble and are completely extracted by water
charged with carbon dioxide. The compounds formed with iron
and aluminum are insoluble in water even in the presence of car-
bon dioxide, but are more or less dissolved by the prolonged
action of organic acids. He endeavored to determine the form
in which phosphoric acid is absorbed. A solution of mono-cal-
cium phosphate was placed in contact with calcium carbonate.
Di- and tri-calcium phosphates were formed depending on the
relative amounts of the materials used. When iron and aluminum
hydroxides were the absorbents, not only was the phosphoric acid
absorbed, but the calcium was also retained. Di-calcium phos-
phate and an iron phosphate were formed. The author concluded
that normal ferric phosphate is formed when sufficient ferric
hydroxide is present. A basic phosphate of iron was also con-
sidered a possibility.

Stocklasa'' estimated the yearly loss of calcium carbonate
from various soils. The results are of interest as bearing on the

1 Bull. Coll. Agr. Tokyo Imp. Univ., 1891

2 I^andw. Versuchs-Stat., 1895, 46. 201.

3 Iiiaug. Dissertation, Dorpat., 1890.

4 Bied. Centr., 1895, 82.

14

loss of phosphoric acid. Four soils — loam, marl, clay and humus
— were anal5^sed and the total phosphoric acid compared with
that found in the drainage water. From the data thus obtained
the annual loss was estimated. The results showed a loss of
9146 grams per hectare in the case of the clay, while the humus
lost 21995 grams per hectare. The humus contained the smallest
per cent of phosphoric acid, but lost the largest amount.

Schreiber^ grew oats followed by turnips on sandy, humus,
and loamy soils using two fertilizers, viz : (i) a mixture of di-
calcium phosphate, calcium sulphate, and magnesium carbonate,
(2) a mixture of sodium phosphate, and carbonates of calcium
and magnesium. The first mixture gave the best results in
every case. The author thought that the diminished action in
the second case was due to the precipitation of phosphoric acid
by the carbonates of calcium and magnesium. His results with
peaty soils showed that the humus phosphoric acid, soluble in
alkaline ammonium citrate, was almost useless for vegetation.
In some cases the humus acted on the assimilable phosphoric
acid in a manner analagous to calcium carbonate.

JofTre^ made vegetation experiments to determine the relative
values of superphosphate, tri-calcium phosphate, and the ferric
phosphate obtained from a sample of manure containing very
little tri-calcium phosphate. Crops of mustard were grown on
plots which differed only with regard to the nature of the phos-
phate present. The ferric phosphate was little better than no
fertilizer, while the other phosphates gave a large increase.

On mixing basic slag and superphosphate with soil, Smora-
wski and Jacobson'* noticed that the phosphoric acid originally
soluble in water was rapidly converted to the citrate soluble
form, and this compound underwent no further change.

Stoklasa'* called attention to the fact that ferrous salts in the
presence of phosphoric acid soluble in water, give rise to the
production of di-tri-ferric phosphate, unless there is an excess of
free phosphoric acid present. With soluble phosphoric acid

1 Exp. Sta. Rec, 1B95, 804.

2 Bull. Soc. Chim., 1896, (3) 15, 42.

3 Bied. Centr., 1896, 580.

4 Ann. Agron., 1897, 23, 588.

15

aluminum salts do not form compounds analagous to those o^
ferrous and ferric salts, but behave like salts of calcium and
magnesium. No data were given.

Prianischnikoff ^ working on the relative values of mineral
phosphates, made sand culture experiments with cereals manured
with phosphorites. The phosphoric acid was only slightly avail-
able. Under the same conditions certain other plants such as
peas, lupines, buckwheat, and mustard seemed to be able to util-
ize the phosphoric acid. The "podzols" (soils containing a
large amount of fine siliceous and organic matter) were appar-
ently able to render phosphorites available for cereals and other
plants. The same was true of forest and peaty soils. Some of
the black soils, however, were without action on the phos-
phorites.

The results of numerous experiments conducted by Ullmann
and Grimm^ showed that for months after the application of
superphosphate, phosphoric acid soluble in water passed through
a depth of soil equal to ten inches. The authors found only a
portion of the magnesia, lime, and oxides of iron and aluminum,
available for retaining phosphoric acid. The mechanical fixa-
tion appeared to depend on the amount of fine sand present,
especially sand of the fineness of dust.

A series of experiments, on the use of lime, extending over
several years, have recently been carried out at the Rhode Island
Experiment Station^. The practical results obtained in field
tests by the use of lime in conjunction with phosphates are of
particular interest, and apparently contradictory to a majority of
the writers on agricultural chemistry. Because of its peculiar
interest, the work is quoted here in some detail. Crops were
grown for four successive years, and the average yields on the
limed and the unlimed plots recorded. The results on the yields
of hay are given herewith as being typical of those obtained
with the crops grown. The figures represent pounds per acre.

1 Ann. Agron., 1899, 25, 177.

2 Chem. Ind., 1900, 23, 61.

3 R. I. Agr. Expt. Sta. Bull., No. 58.

i6

Forms of Phosphoric Acid Applied. lyimed. Unlimed. Gain from

lyiming.

Dissolved Boneblack 19,837 9,820 10,016

Dissolved Bone 19,281 8,564 10,716

Dissolved Phosphate Rock 20,205 8,951 11.253

Fine Ground Bone 22,012 11,855 10, ^57

Basic Slag Meal 20,400 13,193 7,206

Floats 20,525 10,560 9.965

Alumina Phosphate (raw) 14,387 5,042 9,345

Alumina Phosphate (ignited) 19,481 4,93° I4,55i

No Phosphoric Acid 15, 737 2,547 13,190

Double Superphosphate 17, 937 4,752 13,184

It will be seen that there was a wonderful gain in the crop,
in all cases, resulting from the use of lime. The acidulated phos-
phates appeared to be helped more by the lime than the unacidu-
lated forms. The authors suggested that lime might have been
beneficial by correcting the acidity of the first. No experiments,
however, were made to ascertain whether the lime would liberate
phosphoric acid from the insoluble phosphates of the soil.

Some rather peculiar results were obtained by Schreiber^ on
the action of calcium carbonate on mineral phosphates. Addi-
tion of calcium carbonate to the phosphates decreased the action
of the latter on the crops grown, the greater the amount, the
greater the injury. In some cases the effect extended to the
next year. The action of basic slag was not affected.

Kellner and Bottcher'"* obtained similar results, when using
superphosphate, basic slag, and bone meal for oats. Addition of
calcium carbonate caused reduced yields in every case, but this
decrease was less with the superphosphate and slag than with
the bone meal. Comparing the amounts of increase due to phos-
phoric acid applied, the addition of calcium carbonate was not
unfavorable when used in conjunction with slag and super-
phosphate. With bone meal the addition of calcium carbonate
reduced the yields on the average by 67 per cent. No explana-
tion was offered for the action of the latter.

The relation between humus and the phosphoric acid of the
^oil, has not received very much attention. Incidentally, obser-

1 Bied. Centf., igoo, 162.

2 Deut. I,audw. Presse, 1900, 27, 665.

17

vatious have been made on the retentive and solvent powers of
some organic soils, but in most cases the work was designed for
the investigation of other problems.

Fleischer and Kissling\ in a study of the action of moorland
soils on insoluble phosphates, found that the effect was to render
a portion of the phosphate soluble in water, amounting in one
case to 5.5 per cent of the total. At the same time a portion was
reduced to the di-calcium salt, and in one compost as much as 17
per cent of the total was brought into this form. When the ratio
of soil to phosphate was widened, there was an increase in soluble
salt. This increase varied directly with the time of contact.
There appeared to be a limit beyond which the soluble salt became
reduced.

In 1892 Berthelot and Andre^ made some experiments on the
absorption of phosphoric acid by artificial humic anhydride pre-
pared from sugar. Solutions of phosphates of sodium and am-
monium were mixed with the acid anhydride and allowed to
stand in the cold for twenty-four hours. The absorption from
these salts were very small. In most cases practically no phos-
phoric acid was retained.

The action of humic acid in the soils on the solubility of
various natural phosphates was taken up by Minssen and Tacked
Many of the insoluble phosphates were acted on in the presence
of the free humic acid of the soil. When lime was added the
acid was neutralized and the solvent power thereby largely
destroyed. The quantity of phosphoric acid dissolved increased
with an increase in the quantity of free humic acid present. The
numerical agreement, however, was affected by the reabsorption
of the phosphoric acid made soluble.

Snyder* prepared humus by mixing soil with such sub-
stances as clover, flour, straw, saw dust, and sugar, and allowing
to ferment for a year. At the end of that time the humus pro-
duced contained more phosphoric acid than was originally present
in the humus- forming material, indicating that some of the phos-
phoric acid of the soil had united with the humus. By * 'humus'^

1 Bied. Centr., 1883, 155.

2 Ann. Chim. Phys., 27, 196.

3 L,andw. Jahrb., 1898, IV, 392.

4 Minn. Agr. Expt. Sta. Bull., No. 53.

i8

is here meant the soluble organic matter extracted with a 2 per
cent solution of ammonia, after treating the soil with a dilute
solution of hydrochloric acid. The "humic phosphoric acid" is
the phosphoric acid present in the extractive material. Humic
phosphoric acid proved to be one of the most valuble forms.

Dumont^ investigated the abvSorbing power of several organic
soils containing humus and lime. Mono-calcium phosphate was
applied in aqueous solution before and after calcination of the
soil. Calcination seemed to decrease the immediate, but increased
the final absorption of phosphoric acid. These results were con-
strued as proving the combination of humus with phosphoric
acid. The quantities of lime present appeared to have same
effect on the absorption, but the relationship of lime, humus, and
phosphoric acid was not well defined. Mono-calcium phosphate
was also applied to precipitated humus. Absorption took place
in the ratio of humus 10, to P2O5I. Precipitated humus was not
defined.

The use of ferrous sulphate in agriculture, has not been the
object of much experimental enquiry. Griffiths^ however, ob-
tained some very interesting practical results b}^ its use, and his
work is taken up here because of its bearing on the behavior of
phosphoric acid in the soil.

The material was applied at the rate of 50 lbs. per acre.
Among the crops grown were hay, mangels, beans, potatoes, and
wheat. Applied with farmyard manure, its action was injurious,
the poisonous effect being due to the formation of ferrous sul-
phide. The crops in nearly all other cases were increased largely
and the percentage content in phosphoric acid was increased in all
cases. The ferrous sulphate was not oxidized rapidly in the soil,
its presence being proven six weeks after application. Ferrous
sulphate was used in conjunction with superphosphate in all tests.

The subject was taken up a little later by Delacharlony and
Destremx^ They found that the sulphate was productive of in-
creased yields only when the soil contained small quantities of
iron, the increase ceasing when the ferric oxide reached 3 per

1 Compt. rend., 1901, 132, 435.

2 Jour. Chem. Soc, Trans. 1884. 71.— 1886, 114,-1887, 215.

3 Bied. Centr., 1889, 9.

19

•cent, and was detrimental when 4 per cent was reached. The
increase varied with different crops, from 5 per cent to 139 per
cent.

Cazeneuve and Nicolle^ studied the solvent action of ferrous
sulphate on some of the phosphatic fertilizers in common use,
namely, slag, mineral phosphate, bones, superphosphate, and di-
calcium phosphate. After eight days the first three had suffered
no change in the amount of phosphoric acid soluble in acetic acid
and ammonium citrate. With superphosphate half of the water
soluble had become reverted. With the di-calcium phosphate,
the phosphoric acid soluble in acetic acid and ammonium citrate
had increased, but the change was not to the water soluble form.

The action of ferrous sulphate on crops was investigated by
Boiret and PatureP. Seedlings of peas and oats were used in
water culture experiments. Even with very dilute solutions, the
plants were poisoned, owing to the formation of free sulphuric
acid. Iron citrate and citric acid had the same effect. Tests
were then made with soils containing known quantities of lime.
The sulphate was injurious except when there was present a suf-
ficient amount of lime to neutralize the free acid formed. Analysis
of the crops showed that ferrous sulphate had no effect on the
amount of phosphoric acid in the product. In these experiments
the sulphate was not used in conjunction with other fertilizers, as
in the work of Griffiths already quoted.

1 Chem. Centr., IV, (2) 1892, 121.

2 Ann. Agron., 18, 418.

20

SUMMARY

So far as the writer has been able to ascertain, the literature-
cited above includes practically all the work relating directly to
the subject under investigation. Briefly stated, the following
conclusions have been drawn from the work referred to :

I. When soluble phosphates are added to soils the phos-
phoric acid is usually changed to insoluble forms. The action in
most cases is due to chemical change, by virtue of the presence
of compounds of iron, aluminum, and calcium, and possibly
magnesium. These compounds are easily decomposable salts
which can be extracted with hydrochloric acid.

II. Hydrated oxides of iron and aluminum, and carbonate
of calcium unite with phosphoric acid very rapidly. The phos-
phates of iron and aluminum formed are from two to five times as
insoluble in pure water as calcium phosphate. They are also less
soluble in water containing carbon dioxide and various alkali
salts.

III. It is the tendency of phosphoric acid to form the more
difficulty soluble compounds in the soil, but not all the phosphoric
acid appears to be so combined, nor is such combination always
effected with with great rapidity. The complete fixation in
some cases does not take place for several months.

IV. Mixed with calcium carbonate, in the presence of soil
water, phosphates of iron and aluminum undergo decomposition
with the formation of calcium phosphate. With certain soils,
calcium hydroxide also apparently has a solvent action on the
phosphates of iron. Conditions affecting the equilibrium between
these compounds have not been determined.

V. Absorption may be influenced to some extent by the
presence of some of the alkali salts. The effect is not very well
defined. In some cases the solubility of the soil phosphoric acid
seems to be increased in the presence of these salts.

VI. In some cases the humus in soils has the effect of ren-
dering insoluble phosphates more available to plants. The action.

21

seems to be due to free humic acid, rather than to salts of this
acid. This point, however, has not been determined.

VII. Humus is believed to possess great absorptive prop-
erties though experimental evidence supporting this view is
exceedingly limited and not very conclusive. The relation be-
tween humus and phosphoric acid is not well defined.

VIII. Ferrous sulphate in conjunction with phosphates,
applied to some soils is beneficial to crops. It seems to increase
the availability of the phosphoric acid. In large quantities on
acid soils, it acts as a poison to plants. The injurious effect is
due in part to the formation of sulphuric acid. The same effect
may be produced in neutral soils, and soils not sufficiently basic
to neutralize all the acid formed.

IX. Sand and orthoclase offer no resistance to the extrac-
tion of soluble phosphoric acid by pure water.

22

EXPERIMENTAL

1. Methods of Determining Phosphoric Acid.

The work about to be described necessitated a number of
phosphoric acid determinations. Hence it was thought advisible
in most cases to employ the Optional Volumetric Method of the
Association of Official Agricultural Chemists^ In order to test its
accuracy, this method was compared with the Official Gravimetric
Method^ on a variety of materials typical of those to be con-
sidered in the experiments following. The methods were carried
out as follows :

Volumetric Method. The sample was dissolved in from 15
to 30 c.c. of strong hydrochloric acid and from 3 to 10 c.c. of nitric
acid. The solution was cooled, diluted to 250 c.c. , and an aliquot
part taken for the determination. This portion was treated with
10 c c. of nitric acid, nearly neutralized with ammonia, diluted
to about 100 c.c, and heated in the water bath to 65 degrees C.
Molybdic solution was then added (the amount depending on the
amount of PjOg present) the mixture stirred, allowed to stand
thirty minutes, and filtered on an asbestos filter. After thor-
ough washing with cold water, the yellow precipitate was re-
turned to the precipitating beaker, dissolved in standard potas-
sium hydroxide, a few drops of phenolphthalein added, and the
excess of alkali titrated with standard nitric acid. Each cubic
centimeter of the standard potassium hydroxide was equivalent to
I mg. PA-

Gravimetric Method. The sample was dissolved in from
15 to 30 c.c. of strong hydrochloric acid and from 3 to 10 c.c. of
nitric acid. The solution was cooled diluted to 250 c.c, and an
aliquot part corresponding to 0.3 gram taken for analyvSis. The
solution was treated with 10 c.c. of nitric acid, nearly neutralized
with ammonia, diluted to about 100 c.c, and heated to 65 degrees

1 U. vS. Dept. Agr., Div. Cheni., Bui. No. 46, 13 (revised).

2 Ibid., 10.

23

C in a water bath. Molybdic solution was then added, the whole
digested for one hour at 65 degrees C, filtered and washed thor-
oughly with cold water. The precipitate was dissolved on the
filter with ammonia and hot water, knd washed into a beaker to
a bulk of not more than 100 c.c. The solution was nearly neu-
tralized with hydrochloric acid, cooled, and magnesia mixture
added, drop by drop, from a burette, stirring vigorously. After
fifteen minutes, 30 c.c. of ammonia (0.96 sp. gr. ) were added,
and the whole allowed to stand over night. The precipitate was
then collected on a filter, washed with 2.5 per cent ammonia
until free from chlorides, dried, ignited to whiteness and
weighed. The quantity of P2O5 was calculated from the weight
of the Mg^Ffi^, using the factor 0.6396. The following results
were obtained.

Material Grams P2O5

Gravimetric Volumetric

"Insoluble" from Superphosphate 0.0145 0.0147

"Insoluble" from Superphosphate 0.0146 0.0147

Ferric Phosphate 0.1146 0.1137

Ferric Phosphate 0.1149 0.1140

Aluminum Phosphate 0.1137 o. 1143

Aluminum Phosphate 0.1138 0.1146

Calcium Phosphate 0.1361 0.1369

Calcium Phosphate 0.1367 0.1374

The results by the two methods agree closely, the difference
being within the experimental error usually allowed for such
determinations. With the larger quantities of phosphoric acid,
the gravimetric method was used as a check on the volumetric.
The results in all cases, therefore, are strictly comparable.

II. Relation Between Phosphoric Add, and Compounds of Iron,

and Calcium.

Recent results obtained by the use of lime in conjunction
with phosphates of various kinds, have suggested new possibili-
ties with regard to the relation between phosphates of iron and
calcium in the soil. Lime has hitherto been regarded, partly as
a direct plant food, partly as an amendment in liberating plant
food, especially potash, but principally as an agent for improving

24

the physical condition of soils. With regard to the action of phos-
phoric acid in the presence of calcium (lime and calcium carbonate)
and iron, the literature is somewhat contradictory. The reaction
to some extent appears to be a reversible one. The mass law,
which of late years has served to clear up many unexplained
phenomena, undoubtedly plays a very important part.

It occurred to the author that the beneficial effect of lime
might be due, to some extent, to the chemical action of this sub-
stance in rendering phosphates more available to crops, and that
the behavior of phosphoric acid in the presence of compounds of
calcium and iron, might depend on the relative quantities of the
last two present. The experiments designed to study these
points are described under the following heads :

(a) Action of lime on ferric phosphate.

(b) '* " " " the phosphate formed by fixation.

(c) ''• " '• *' soils.

(d) Fixation of phosphoric acid in the presence of com-

pounds of iron and calcium.

(a) ACTION OF LIME ON FERRIC PHOSPHATE.

The object of the experiment was to determine the effect of
lime on the availability of precipitated and natural ferric phos-
phates.

♦ The solvent selected for this purpose was the i per cent citric
acid solution proposed by Dyer\ for the determination of avail-
able plant food in soils. This reagent is the only one of the
many solvents proposed for determining available plant food, that
has received any favor among agricultural chemists. While it is
still of doubtful efficiency in determining absolute availability, in
determining relative values, for which purpose it was used in the
present work, it has been of unquestioned service. Dyer pro-
posed this reagent as representing the average root juice acidity
of a large number of agricultural plants examined by him.

Precipitated ferric phosphate was prepared by treating fer-
ric chloride with di-sodium phosphate. The resulting precipitate
was washed thoroughly with cold water and the product air

I Jour. Cheni. Soc. Trans., 1894, 115.

25

dried. Phosphoric acid was determined by the gravimetric
method, and ferric oxide by the vohimetric bi-chromate method.
The following results were obtained :

Fe,0, 41.53%

P2O5 38.20%

H2O 20.08%

Correcting for hygroscopic moisture, the figures agree with
the formula FePO,.2H20.

The only natural ferric phosphate that could be obtained
was a sample of Dufrenite containing 16.01 per cent of P2O5.
This is a basic ferric phosphate.

The solubility of these materials was first determined by
placing I gram in a small flask, adding 100 c.c. of i per cent
citric acid solution, and allowing to stand twenty-four hours at
room temperature, shaking once each hour. The insoluble resi-
due was then filtered off, washed with cold water, and the phos-
phoric acid determined. The following results were obtained :

Material.

Precipitated Ferric Phos

Precipitated Ferric Phos

Dufrenite

Dufrenite

The effect of lime on the solubility of these phosphates was
then determined. For this purpose i gram of each of the ma-
terials was placed in a flask, pure slaked lime (free from phos-
phoric acid) added, and enough water to make a thin paste.
After twenty-four hours the contents of the flask were treated
with 100 c.c. of I per cent citric acid solution and an additional
amount of the crystallized acid to neutralize the excess of lime.
The solubility in the citric acid was then determined as in the
first instance. The following table contains the results.

P2O5 sol. in

Per cent of

r cent citric acid.

total.

12.20%

31.9

12.05%

31-5

0.07%

0.4

0.09%

0.5

cent citric acid.

total.

24.95%

65.3

24.90%

65.2

35.80%

93.7

35.80%

93-8

37.00%

96.9

36.90%

96.6

0.45%

2.8

0.47%

2.9

26

Material. I,ime. P2O5 sol. in i per Per cent of

Prec. Ferric Phos i gram

Prec. Ferric Phos i gram

Prec. Ferric Phos 0.5 gram

Prec. Ferric Phos 0.5 gram

Prec, Ferric Phos 0.25 gram

Prec. Ferric Phos 0.25 gram

Dufrenite i gram

Dufrenite i gram

The following table shows the comparative solubility of the
phosphoric acid (PgOJ in i per cent citric acid, with and without
the addition of lime :

Material. Without lime. With lime.

I gr- 0.5 gr. 0.25 gr.

Prec. Ferric Phos. __ 12.20% 24.95% 35.80% 37.00

Prec. Ferric Phos. __ 12.05% 24.90% 35-90% 36.90

Dufrenite 0.07% 0.45%

Dufrenite 0.09% 0.47%

It is seen from the above tables that the action of lime on
the natural ferric phosphate was not very marked. In the case
of the precipitated ferric phosphate the lime caused a considerable
increase in the solubility. The larger quantities did not cause
as great an increase as the smaller quantities. This fact indi-
cated the formation of the more soluble calcium phosphates with
the smaller quantities. With an excess of lime, as in the experi-
ment in which i gram was used, tri-calcium phosphate was
probably formed, and this compound is much less soluble in i
per cent citric acid than the di- and mono-calcium compounds.

It is to be remarked that the reaction was fairly rapid.
Within one hour after the addition of lime, a red compound began
to separate. This compound was probably ferric hydroxide. An
attempt was made to separate the products formed in the reaction.
Various solvents were tried, but it was found impossible to effect
a separation of calcium and ferric phosphates in the presence of
ferric hydroxide. In the course of these trials a 5 per cent solu-
tion of neutral potassium oxalate was observed to act on precipi-
tated ferric phosphate, dissolving both the iron and the phos-

27

phoric acid in the proportion in which they existed in the
original compound. This action of potassium oxalate was made
the basis of a qualitative test for the presence of ferric phosphate,
in the presence of ferric hydroxide. The solution was found to
have no solvent action on ferric hydroxide. The compound
formed with ferric phosphate appeared to be a double oxalate,
very soluble in water.

Ferric phosphate was treated with varying quantities of
lime. The products of the reaction were treated with the solu-
tion of potassium oxalate, filtered, and the filtrate tested for
iron. The absence of iron in the oxalate solution was taken as
showing the absence of ferric phosphate in the mixture treated
with this solvent. 0.2 gram lime was found to be the smallest
amount capable of completely converting i gram of ferric phos-
phate.

The behavior of potassium oxalate suggested the use of this
reagent for determining the available phosphoric acid in phos-
phatic materials. A few preliminary trials on its solvent action
on calcium and iron phosphates, under different conditions, were
made. The results were not sufiiciently satisfactory to indicate
the usefulness of this reagent.

(b) ACTION OF LIME ON THE PHOSPHATE FORMED BY FIXATION.

It is fairly well established that soluble phosphates when
applied to soils tend to pass into comparatively insoluble forms.
This reversion is caused largely by hydrated oxide of iron. It
was considered important therefore, to know the effect of lime on
the compound thus formed.

To this end two grams of pure ferric hydroxide were treated
in a small flask with 50 c.c. of an aqueous solution of superphos-
phate, shaken, and allowed to stand ten days with occasional
agitation. The insoluble residue was then filtered off and washed
with cold water until phosphoric acid ceased to be given up.
The filtrate and washings were then analyzed and the phos-
phoric acid absorbed calculated. Several residues were obtained
in this way. Two of these were treated with 100 c.c. of i per
cent citric acid solution, and the solubility determined as de-

28

scribed under ferric phosphate. Four others were treated with
0.5 gram of lime, enough water added to make a thin paste, and
the mixture allowed to stand one and nine days. The solubility
in I per cent citric acid was determined, sufficient crystallized
citric acid being added to correct the basicity due to lime. The
superphosphate solution originally added contained 0.1340 gram
of P2O5. Under "residues" in the following table are included
the amounts of PgOg absorbed :

P2O5 in I,ime added. P2O5 sol. in Per Cent

residues. i per cent citric acid. soluble.

0.1249 grs. 0.0214 grs. 17. 1

0.1248 grs. 0.0224 grs. 17 9

0.1244 grs. o. 5 grs. one day 0.0544 grs. 43.7

0.1240 grs. 0.5 grs. one day 0.0535 grs. 43.0

0.1244 grs. 0.5 grs. nine days 0.0499 grs. 40.1

0.1244 grs. 0.5 grs. nine days 0.0495 grs. 39.7

Incidentally a similar experiment was made using neatral
ammonium citrate solution (1.09 sp.gr.) as the solvent. For
various reasons this solvent was not used in the soil investiga-
tions. The preceding plan was followed except that the action
of lime was allowed to continue for two days. The results were
as follows :

P2O5 in I^ime added. P2O5 sol. in Per Cent

residues. am. citrate. soluble.

0.1217 grs. 0.0535 grs. 43.9

0.1211 grs. 0.0534 grs. 44.0

0.1223 grs. 0.5 grs. two days 0.0536 grs. 43.8

0.1224 grs. 0.5 grs. two days 0.0527 grs. 43.0

The results show that lime materially increased the solubility
of reverted phosphate of iron in i per cent citric acid. The action
appeared to be complete in one day. The solubility of the re-
verted phosphate in ammonium citrate solution was not increased
by the addition of lime.

The composition of this phosphate of iron has not been deter-
mined. It is much more insoluble in i per cent citric acid, and am-
monium citrate than the normal ferric phosphate. That none of
the latter compound was formed in the fixation experiment, was
indicated by the following tests :

29

( I ) . Ferric hydroxide and a solution containing phosphoric
acid in large excess of the amount necessary to form the normal
salt, were allowed to react for weeks. The amount of phosphoric
acid absorbed was then determined.

Fe(0H)3 used 0.2500 grs.

P2O5 absorbed 0.0275 "

P2O5 required to form FePO^ 0.1664 "

Even in the presence of a large excess of phosphoric acid,
the ferric hydroxide absorbed little more than one tenth of the
amount necessary to form the normal salt.

(2). The compound formed was treated with the 5 per
cent solution of potassium oxalate already described. After
filtering the filtrate was tested for iron, the absence of which
showed the absence of normal ferric orthophosphate.

The compound formed by fixation is evidently a highly basic
phosphate of iron. The compound appears to be formed even
when only a limited amount of the base is present. In the soil,
the bases would usually be present in large excess with respect
to the phosphoric acid.

In the experiments just described, the lime was brought into
intimate contact with the phosphate of iron. Its action there-
fore was probably at a maximum. It was thought advisable to
supplement this with artificial soil experiments under conditions
which usually obtain in soils, conditions presumably less favora-
ble to the action of lime.

For this purpose 200 gram portions of clean sand were ex-
tracted with hydrochloric acid and washed free from acid with
cold water. A blank experiment showed that the product pos-
sessed no absorptive properties for phosphoric acid. Each por-
tion was mixed with 4 grams of ferric hydroxide and placed in a
six inch cylinder. Superphosphate solution was then added and
absorption allowed to continue for ten days. During the mean-
time the mixture was kept moderately moist. After ten days
the phosphoric acid unabsorbed was washed out with cold water
and determined, the amount absorbed being calculated from the
data thus obtained. The mixture then represented a soil con-
taining 1.5 per cent Fe.Pg, and 0.12 per cent of P2O5.

30

For this work and the soil investigation to be described later
two solvents were selected, namely, i per cent citric acid solu-
tion, and hydrochloric acid. The first has already been re-
ferred to , but for soil work the conditions attending its use were
modified somewhat.

Hydrochloric acid was recently proposed by Moore^ for

the determination of available mineral plant food in soils. In the
work referred to pot experiments were made with a large variety
of soils, and the amounts of phosphoric acid extracted by the
solvent in question agreed closely in nearly all cases with the
actual amount removed by the crops.

The solubility determinations with the reagents selected
were carried out in the following manner. The ratio of solvent to
substance was 500 c.c. to 100 grams. These were placed in a
litre glass stoppered bottle, and kept at a temperature of 40 de-
grees C in a water bath for exactly five hours. The bottles were
shaken every fifteen minutes. After the digestion the whole was
shaken and emptied on to a folded filter sufficiently large to hold
the entire contents of the bottle. After draining, 400 c.c. of the
filtrate representing 80 grams of the soil, were evaporated to
dryness. In the case of the citric acid the residue was treated
with magnesium nitrate solution, evaporated to dryness, and
ignited until all organic matter was destroyed. These residues
were taken up with water and nitric acid and the phosphoric
acid determined.

In all the experiments a preliminary digestion was made
with 20 grams of soil and 100 c.c. of the solvent, in order to
determine the basicity of the soil. A correction was then made
in the strength of the acids so as to reduce the solvent action to
a uniform basis.

Portions of the artificial soil described were air-dried and the
solubility of the phosphoric acid determined. Lime (one and
two grams) was mixed with other portions and the action allowed
to go on for seven days, the mixture being kept moist during the
meantime. These mixtures were then air-dried and solubility
determinations made. The following results were obtained.

I Jour. Am. Chem. Soc, 1902, 79.

31

SOLVENT — I PER CENT CITRIC ACID SOLUTION

P2O5 in I^ime added. P2O5 sol. in Percent

residues. i per cent citric acid. soluble.

0-2530 grs. 0.0622 grs. 24.5

0.2540 grs. 0.061 1 grs. 24.0

0.2488 grs. I gram 0.1060 grs. 42.6

0.2483 grs. I gram 0.1070 grs. 43.0

0.2527 grs. 2 grams 0.1232 grs. 48.7

0.2517 grs. 2 grams 0.1220 grs. 48.4

SOLVENT — HYDROCHLORIC ACID

200

P2O5 in Lime added. P2O5 sol. in Per Cent

residue. i per cent citric acid. soluble.

0.2530 grs. 0.0033 grs. 1.3

0.2540 grs. 0.0038 grs. 1.4

0.2488 grs. I gram 0.0044 S^^- i-7

0.2483 grs. I gram 0.0052 grs. 2.0

0.2527 grs. 2 grams 0.0048 grs. 1.8

0.2517 grs. 2 grams 0.0052 grs. 2.0

From the results it is seen that lime increased the solubility
in I per cent citric acid considerably as in the preceding experi-
ments. The increase is somewhat less, however, with the arti-
ficial soil.

With regard to the solubility in hydrochloric acid, the

addition of lime produced little change. The differences are
within the range of experimental error. It is to be remarked
that the solvent action of the hydrochloric acid was very small.

(c) ACTION OF LIME ON SOILS.

For the purpose of studying the effect of lime on the phos-
phoric acid originally present in soils, two samples of the latter
were taken. The soil designated "A" was a sandy loam under
cultivation. Soil "B" was a clay loam not under cultivation.

Samples were prepared for analysis according to the methods
of the Association of Official Agricultural Chemists\

I U. S. Dept. Agr„ Div. Chem., Bui. No. 46, 71.

32

The determination of total phosphoric acid was made,, the
method being to weigh out 2 grams of soil into a platinum dish
and ignite to drive off organic matter. Put in i or 2 c.c. of
hydrofluoric acid and allow the soil to come in contact with the
acid slowly to avoid loss by sputtering. After the violent action
has ceased, place on a water bath and evaporate to dryness. After
repeating this operation once or twice, take up with a little nitric
acid and water and determine phosphoric acid. This method
was rapid and gave closely agreeing results. In this way the
following results were obtained :

Soil "A." Soil "B."

Total F,0, o. 145% o. 155%

Complete analyses of the acid soluble materials in these were
then made according to the methods of the Association of Official
Agricultural Chemists\ with the following results :

DIGESTION IN HYDROCHI^ORIC ACID

Constituent.

Insoluble matter

Potash (K2O)

Soda (NaaO )

Lime (CaO)

Magnesia ( MgO)

Oxide of Manganese (MnO)

Ferric Oxide (FcgOg)

Alumina (AI2O3 )

Phosphoric Acid (P2O5)

Sulphuric Acid (SO3)

Carbonic Acid (CO2)

Volatile matter

Portions of 200 grams of the air dried soils were then mixed
thoroughly with lime (one and two grams), the whole placed in
cylinders, moistened and allowed to stand exposed to the atmos-
phere for seven days. The portions were then again air dried,
and weighed. The solubility of the original and limed portions

in I per cent citric, and hydrochloric acids was then deter-

200

ID I. 115 SP.

GR.

Soil "A."

Soil "B.'

87-85%

83-53%

0.74%

0.80%

0.20%

0.41%

0.11%

0.14%

0.55%

0.78%

0.03%

0.02%

1.74%

1.91%

4.00%

5-72%

0.12%

0.10%

0.04%

0.06%

0.26%

0.18%

4.17%

6.51%

U. S. Dept. Agr. Div. Chem., Bui. No. 46, 72.

33

mined according to the method described under artificial soils,
the quantities taken being calculated on a basis of the original
air dry soil. The quantities of phosphoric acid in the following
tables are calculated to this basis, and are therefore comparable
in all cases.

SOIL "a." solvent— I PER CENT CITRIC ACID

Litne added.

None

None

One gram

One gram

Two grams

Two grams

;05 soluble.

Per cent of

total.

0.028%

19.3

0.030%

20.7

0.031%

21.3

0.032%

22.0

0.037%

25.5

0.038%

26.2

SOIL B.

SOLVENT — I PER CENT CITRIC ACID

Lime added. P2O.5 soluble. Per cent of

total.

None 0.0068% .4.3

None 0.0067% 4-3

One gram 0.0132% 8.5

One gram 0.0130% 8.3

Two grams 0.0100% 6.4

Two grams 0.0102% 6.5

SOIL "A." SOLVENT HYDROCHLORIC ACID

200

Lime added. P2O5 soluble. Per cent of

total.

None 0.0030% 2.0

None 0.0030% 2.0

One gram 0.0043% 2.9

One gram 0.0041% 2.8

Two grams 0.0040% 2.8

Two grams 0.0038% 2.7

SOIL B." SOLVENT HYDROCHLORIC ACID

200

The amount of phosphoric acid was too small to be deter-
mined. The addition of lime (one and two grams), seemed to
produce no effect on the solubility of the phosphoric in this soil.

34

The two soils used in the experiments described contained
practically equal amounts of total phosphoric acid, and phosphoric
acid soluble in hydrochloric acid 1.115 sp. gr., while the avail-
bility of that in the cultivated soil, as shown by the weak acid
solvents, was about five times as great as that in the uncultivated
soil. The chemical analyses of the two present no differences
that could serve as an explanation of the difference in availability.
This difference, therefore, is probably due, partly to the state of
cultivation, and partly to the physical characteristics of the soils.

From the results obtained it is doubtful whether hydro-

200

chloric acid would prove a suitable solvent for the determination

of available plant food in all soils. Comparing the two soils,

however, the results with it indicate in a general way the same

relative availability of the phosphoric acid as is shown by the

solubility in i per cent citric acid.

In regard to the action of lime, the solubility in citric acid
was increased slightly in both soils. The solubility in hydro-
chloriQ acid was not increased in the case of the uncultivated
soil. This soil evidently held the phosphoric acid in a very
insoluble form.

It is interesting to note the difference in action of lime on
natural and artificial soils. A comparison is made in the follow-
ing table.

SOLUBILITY OF PjOg IN I PER CENT CITRIC ACID

Without lime. With lime.

Soil ''A" 19.3% 21.3%

Soil "A" 20.7% 22.0%

Artificial soil 17.1% 43-7%

Artificial soil i7-9% 43-o%

The figures represent per cent of the total phosphoric acid .
It is evident that the natural soil was less susceptible to the
action of lime than the artificial product.

(d) FIXATION OF PHOSPHORIC ACID IN THE PRESENCE OF
COMPOUNDS OF IRON AND CALCIUM

While the tendency of soluble phosphates, applied to soils
is to pass into the insoluble phosphate of iron, several investi

35

gators, notably Deherain, KostitchefF, and others have observed
that calcium carbonate reacts with ferric phosphate forming cal-
cium phosphate, the action taking place even in the presence of
an excess of ferric hydroxide. Whether this was the reverted or
the normal ferric phosphate was not stated.

It occurred to the author that the presence of a relatively
large quantity of calcium compounds might interfere with the
formation of reverted phosphate of iron. A short experiment
was made in order to ascertain whether the presence of calcium
carbonate would influence the fixation of phosphoric acid by
ferric hydroxide.

Tri-calcium phosphat^ was dissolved in water charged with
carbon dioxide, and 200 c.c. of the solution containing 0.0176
gram of PjOg were added to i gram of ferric hydroxide, with and
without varying quantities of calcium carbonate. The action was
allowed to continue for ten days, after which the mixtures were
filtered. The solubility of the residues in ammonium citrate solu-
tion was determined by adding 50 c.c. and allowing to stand
twenty-four hours in the cold. This residue was filtered and
washed with cold water. The results are contained in the fol-
lowing table :

Material mixed with 0.0176 grs. P2O5. P2O5 insoluble in

ammonium citrate.

I gram CaCOg 0.0018 grs.

I gram Fe( OH) ^ 0.0145 grs.

I gram Fe(0H)3 plus i gram CaCOa 0.0152 grs.

I gram Fe(0H)3 plus 2 grams CaCOg 0.0147 grs.

In the blank determination above practically all the phos-
phoric acid was extracted by ammonium citrate. The presence
of calcium carbonate (one and two grams) apparently had no
effect on the solubility of the compound formed by the reversion
of phosphoric acid.

III. Behavior of Phosphoric Acid towards Humus

That humus possesses the power of absorbing phosphoric
acid from solution has not been proven. Organic soils have been
known to effect solution of insoluble phosphates in some cases,

36

while in other cases soils containing a large amount of humus ex-
hibited a strong attraction for phosphoric acid.

In studying the retentive power of humus for phosphoric
acid it was thought best to work with the artificial preparation,
thereby eliminating the possibility of the interference of any other
absorbing material.

To this end humic acid was prepared from sugar according
to the method described by Berthelot\ 600 grams of sugar fur-
nished 100 grams of the air dry material. In one portion the
moisture was determined by drying to constant weight at 100 de-
grees C. Other portions were subjected to combustion analysis.
The following analysis is calculated on the basis of dry matter.

Carbon 61.48%

Hydrogen 4.74%

Oxygen 33-78%

The phosphoric acid to be absorbed was applied in the form
of an aqueous extract of superphosphate, thus approximating
conditions which obtain in actual practice. Portions of i gram
of the air dry humic acid were placed in a small flask, 50 c.c. of
a solution containing 0.1550 gram PgOg and 0.0185 gram calcium
added to each, and the mixtures allowed to stand one, four and
eight days with occasional shaking. At the end of these periods
the insoluble matter was filtered off and washed with cold water
until the wash water gave no test for phosphoric acid. The
quantities of phosphoric acid and calcium in the filtrate and
washings were then determined. Calcium was determined by
the volumetric permanganate method. The results are found in
the following table.

No. Duration of contact. P2O5 soluble. Ca soluble.

1 one day o.i540grs. 0.0173 grs.

2 one day 0.1545 grs. 0.0180 grs.

3 four days 0.1535 grs. 0.0185 grs.

4 four days o.i550grs. 0.0188 grs.

5 eight days 0.1550 grs. 0.0180 grs.

6 eight days 0.1550 grs. 0.0177 grs.

As is shown by the above results, neither the phosphoric
acid nor the calcium were retained by the insoluble humic acid.

I Chim. Veg. Agr., 4, 123.

37

It was not expected that chemical absorption would take place.
The experiment was suggested by the statements of many
writers to the effect that humus is active in retaining plant food
by virtue of its physical structure. The data given above indicate
that insoluble free humic acid exerts no influence in rendering
phosphoric acid insoluble in water.

The organic acids present in the soil do not, however, exist
to any great extent in the free state except in bogs and morasses.
Soils suitable for agricultural purposes contain little or no free
acid except carbonic acid. In most cases they give an alkaline
reaction. Humic acid exists as humates of the bases predomi-
nating in the soil. These compounds may be divided into two
classes, namely, (i) those insoluble or but slightly affected by
water, including humates of calcium, magnesium, iron, and
aluminum, and (2) those soluble in water including humates of
the alkalies. Representative of the first class, calcium humate
was selected for study.

This compound was prepared by mixing 10 grams of the
humic acid already described with one litre of lime water con-
taining 2.42 grams of CaO, and allowing the mixture to stand
for five days with occasional shaking. The insoluble residue
was then filtered off and washed with cold water until the wash-
ings were neutral. On being subjected to combustion analysis
the product showed the following composition, calculated on the
basis of dry matter.

Carbon 55-48%

Hydrogen 4. 1 1 %

Oxygen 34.8i%

Calcium 5.60%

This preparation was treated with a solution of superphos-
phate in the manner already described for humic acid. The solu-
tion contained,

P2O5 o. 1550 grs.

Ca 0.0185 grs.

The quantities of phosphoric acid remaining unabsorbed by i
gram of the calcium humate are given below.

38

Duration of contact. P2O5 soluble. Ca soluble, P2O 5 absorbed.

One day o. I385grs. 0.0309 grs. 0.0165 grs.

One day 0.1385 grs. 0.0312 grs. 0.0165 grs.

Four days 0.1360 grs. 0.0278 grs. 0.0190 grs.

Four days o. 1365 grs. 0.0283 grs. 0.0185 grs.

Eight days 0.1340 grs. 0.0259 grs. 0.0210 grs.

Eight days 0.1345 grs. 0.0260 grs. 0.0205 grs.

The above figures show that absorption of phosphoric acid
took place to a limited extent. It occurred to the author that a
small amount of calcium carbonate might have been formed in the
preparation of the calcium humate, in which case the absorption
could not be ascribed to the latter compound. Carbon dioxide
was determined in a portion of the preparation therefore, the
amount obtained being 0.64 per cent. Calculated to calcium
carbonate the one gram of the material contained 0.0142 gram of
the material contained 0.0142 gram of the carbonate, which in
turn would be sufficient to react with 0.0207 gram of PgOg form-
ing the mono-calcium salt. This salt is decomposed by water with
the formation of the di-calcium phosphate which is not soluble in
water. The absorption therefore is to be referred to the presence
of calcium carbonate.

The increase of the calcium in solution was due to the for-
mation of some of the mono-calcium phosphate from the free
phosphoric acid present in the superphosphate solution.

It is to be inferred from these facts that calcium humate does
not take part in the absorption of phosphoric acid from super-
phosphate.

As the majority of the humus compounds found in the soil
contain nitrogen, it was thought advisable to include some of
these in the present work. The compounds used were soluble
and insoluble ammonium humate.

For the preparation of these 25 grams of humic acid were
treated with 500 c.c. or ammonia solution containing 14 3 grams
of ammonia, the flask closed, shaken and allowed to stand five
days. The insoluble residue was then filtered off and washed
until free from ammonia. When dried the product weighed 1 1
grams showing that a little more than half of the humic acid had
been dissolved by the ammonia. The soluble portion was put

39

into an evaporator and allowed to stand in the cold until all free
ammonia had evaporated. The solution was reserved for further
investigation.

The insoluble portion was subjected to a combustion analy-
sis. The "nitrogen as ammonia" represents the amount of
nitrogen expelled by boiling with magnesia.

Carbon 62.29%

Hydrogen 5-62%

Oxygen 28.81%

Nitrogen (total) 3-28%

Nitrogen (as ammonia) 1.08%

Although humus varies in its content of nitrogen (depend-
ing on the material from which it is formed) the above agrees
fairly well with the average analysis of humus extracted from
soil by various solvents. Regarding the form in which nitrogen
is held in this compound, Berthelot has shown that besides
ammonia, it is present most probably as amide. This fact is in
accordance with the data obtained when working with the nat-
ural product.

The action of this substance on superphosphate was investi-
gated. The material was treated with a solution of superphos-
phote in the manner already described for humic acid. The
solution contained,

P2O5 o. 1550 grs.

Ca i. 0.0185 grs.

The quantities of phosphoric acid remaining unabsorbed by
I gram of the insoluble ammonium humate are given below.

No. Duration of contact. P2O 5 soluble. Ca soluble.

1 one day 0.1540 grs. 0.0185 grs.

2 one day 0.1545 grs. 0.0178 grs.

3 four days 0.1540 grs. 0.0191 grs.

4 four days 0.1540 grs. 0.0183 grs.

5 eight days 0.1540 grs. 0.0178 grs.

6 eight days 0.1540 grs. 0.0187 grs.

The results show that the insoluble ammonium humate ex-
erted no influence in retaining the phosphoric acid of superphos-
phate. The content in calcium was also not affected.

40

The soluble ammonium humate already referred to contained,

Nitrogen (total) 20.25%

Nitrogen (as ammonia) 9-65,%

calculated on a basis of dry matter. With a solution of super-
phosphate, this substance produced a brown flocculent precipitate.
Thinking that this precipitate consisted of calcium humate,
another portion of the ammonium humate solution was treated
with lime water. No precipitate was formed. Neither calcium
chloride or ferric chloride produced any change. On adding
pure orthophosphoric acid a brown precipitate was formed appar-
ently identical with that formed with the superphosphate solu-
tion. These facts indicated the formation of an organic com-
pound containing phosphoric acid. An attempt was made to
separate this compound, but it was exceedingly difficult to wash,
and apparently underwent partial decomposition.

From the foregoing data it appears that insoluble humus offers
no resistance to the extraction of the phosphoric acid of super-
phosphate by water. The soluble humus appears on the other
hand to form an insoluble compound with phosphoric acid, and
thus acts as an absorbent. To what extent such absorption may
take place was not determined.

(b) SOLUTION

The solvent action of two of these humus compounds was
considered briefly. For this purpose precipitated tri-calcium
phosphate, humic acid, and soluble ammonium humate were
selected. The experiments were carried out as follows.

Portions of 0.5 gram of pure precipitated tri-calcium phos-
phate were placed in small flasks, and to some of these was added
I gram of insoluble humic acid, and 10 c.c. of water. To other
portions were added 10 c.c. of the solution of ammonium humate.
The contents of the flasks were shaken occasionally for five days,
after which the residues were filtered off, washed with cold
water, and the phosphoric acid determined in the filtrate and
washings.

A blank experiment was made with 0.5 gram of precipitated
tri-calcium phosphate and 10 c.c. of water treated as described

41

for the other experiments. The results of these tests are given
in the following table.

Material. P2O5 soluble in water.

Tri-calcium phos, 0.5 gr none

Tri-calcium phos. 0.5 gr none

Tri-calcium phos. 0.5 gr. am. humate 10 c.c._ none

Tri-calcium phos, 0.5 gr. am. humate ioc.c._ none

Tri-calcium phos. 0.5 gr. humic acid i gr 0.0030 gr.

Tri-calcium phos. 0.5 gr. humic acid i gr. 0.0028 gr.

The ammonium humate was without action on the tri-
calcium phosphate. The free humic acid rendered a portion of
the phosphoric acid soluble in water, though the action was not
very great. These facts are in accordance with the observations
of other workers, namely, that humus is active in rendering
insoluble phosphoric acid available to plants. The results pre-
sented herewith indicate that this action is due to the humic
acid and not to the compounds of humic acid, namely, the
humates.

IV. Absorption of Phosphoric Acid by Zeolites

There is abundant evidence to show the existence in soils of
easily decomposable silicates analagous to the zeolites. The
power of these substances to fix the bases potassium, sodium,
calcium, ammonia, and magnesium has been thoroughly investi-
gated by a number of workers.

The work of Doelter (i), Friedel (2), Rinne (3), Clarke (4),
and others on the constitution and properties of the zeolites, sug-
gested to the writer the possibility of the power of these sub-
stances to absorb phosphoric acid.

For this investigation both the artificial and natural silicates
were used. The first was prepared by treating a solution of soda
alum with a solution of sodium silicate. The resulting precipi-
tate was washed thorougly with cold water, filtered and dried.
The product was difficult to wash, and seemed to undergo partial
decomposition even in cold water, with the elimination of sodium
silicate. The dry material was found to contain the following :

42

Alumina i8. 12%

Silica 57-21%

Soda 5-85%

Water 18.82%

Two grams of this substance were treated with superphos-
phate solution, allowed to stand seven days, filtered, washed, and
the phosphoric acid in solution then determined. The residues
were treated with 100 c.c. of i per cent citric acid solution and
allowed to stand for twenty-four hours. The phosphoric acid
dissolved by this reagent was then determined, with following
results :

No. P2O 5 absorbed. Percentsol.ini

per cent citric acid,

1 0.1340 grs. 56.8

2 0.1340 grs. 57.4

The natural silicates used were selected with a view of ob-
taining as much variety in composition as possible. A few insol-
uble silicates were also used.

It is not within the province of this paper to enter into a
discussion of the constitution of these compounds. The empir-
ical formulas are given herewith.

Analcite ^ NaAlSizOg H2O

Chabazite (CaNag )Al2Si40i2 6H2O

Halloysite H4Al2Si209 aq.

Heulandite H4CaAl2(Si03)6 3H2O

Prehnite H2Ca2Al2(Si04)3

Pyrophyllite H2Al2(Si03)4

In order to determine the absorptive power of these sub-
stances for phosphoric acid, 2 gram portions of each were placed
in small flasks, and to each were added 50 c.c. of a solution of
superphosphate containing o. 1550 gram P2O5. The experiment
was carried out in the manner described for the precipitated sil-
icate. Solubility determinations of the residues, in i per cent
citric acid were made in a similar way. A check determination
was made with washed sand. The following table contains the
data obtained.

43

Material. P2O5 absorbed. Per cent sol. in i

per cent citric acid.

Sand none

Analcite 0.0305 grs. 46.5

Analcite 0.0295 grs. 44.7

Chabazite 0.0170 grs. loo.o

Chabazite 0.0165 grs. lOo.o

Halloysite / 0.0125 grs. 80.8

Halloysite 0.0125 grs. 80.8

Heulandite 0.0130 grs. loo.o

Heulandite 0.0125 grs. loo.o

Prehnite 0.0125 grs. 77.6

Prehnite 0.0120 grs. 76.6

Pyrophyllite none

The results show that some of the natural silicates (zeolites)
possess an absorbtive power for phosphoric acid, while the arti-
ficial preparation did. so to a marked degree. The compounds
formed appeared to be more soluble in i per cent citric acid than
the compound formed when ferric hydroxide was the absorbent.

The nature of this absorption is not understood. In the light
of our present knowledge of the zeolites present in the soil it was
considered impracticable to undertake to ascertain its nature.

It occurred to the writer that this absorption might be due
to the presence of free hydrated oxide of aluminum, in which case
the action could not properly be referred to the silicates in ques-
tion. In order to determine this point, some representative sam-
ples of the materials used were strongly dehydrated over the
blast lamp and the absorptive power again determined with the
following results :

Material. Before dehydration After dehydration.

P2O5 absorbed.

Analcite 0.0305 grs. 0.0348 grs.

Analcite 0.0295 grs. 0.0348 grs.

Chabazite 0.0170 grs. 0.0170 grs.

Chabazite 0.0165 grs. 0.0175 grs.

Precipitated siHcate o. 1340 grs. o. 1 170 grs.

Precipitated silicate o. 1340 grs. o. 1 170 grs.

With analcite dehydration seemed to increase the absorptive
power though only to a slight extent. Dehydration of the arti-

44

ficial preparation decreased slightly the absorptive power, indi-
cating the presence of a small amount of aluminum hydroxide.

It is doubtful whether there exists in soils silicates identical
either with the natural or the artificial materials used in the
above work. But it is believed that the data here presented
serves to show that the zeolitic silicates in soils may take part
in the fixation of phosphoric acid.

V. Action of Ferrous Sulphate

The value of ferrous sulphate in agriculture appears to be
questionable. Regarding its action towards phosphoric acid,
two possibilities suggested themselves, namely, (i) the hydrol-
ysis and consequent solvent action of the sulphuric acid formed,
on the insoluble phosphates, (^2) the action in retaining the
phosphoric acid applied in a comparatively soluble form.

The solvent effect on several phosphatic materials was deter-
mined by placing i gram of each in a flask and treating with
I gram of ferrous sulphate, 100 c.c. of water, and shaking
occasionally for four days. The residues were filtered, washed,
and the filtrate tested for phosphoric acid. None was present in
any case. The residues were then treated with 100 c.c. of i per
cent citric acid, and the solubility of the phosphoric acid in this
reagent determined. The following table contains the data
showing the effect of ferrous sulphate on the solubility in i per
cent citric acid.

SOLUBILITY IN I PER CENT CITRIC ACID

Material.

Tri-calcium phosphate

Tri-calcium phosphate

Ferric phosphate

Ferric phosphate

Apatite

Apatite

Wavellite

Wavellite

No FeS04.

FeS04

O5 dissolved.

P2O5 dissolved.

26.40%

20.40%

26.20%

20.50%

12.05%

ir.50%

12.20%

11.25%

1.47%

1.36%

1.47%

1.44%

trace

trace

trace

trace

45

Ferrous sulphate did not increase the citric acid solubility of
the phosphates under investigation. In fact with the tri-calcium
the effect was to lessen the solubility.

The remarkable effect of ferrous sulphate when used with
superphosphate as was the case in the field tests made by Grif-
fiths, indicated that the substance converted the phosphoric acid
into forms more available than the reverted phosphate formed
from ferric hydroxide.

The precipitate formed on the addition of superphosphate to
ferrous sulphate was found to be easily soluble in i per cent citric
acid. In order to determine whether this compound is formed in
the presence of ferric hydroxide, 2 grams of the latter were
mixed with 0.5 gram of ferrous sulphate and 50 c.c. of super-
phosphate solution. Of the reverted phosphate of iron formed
from ferric hydroxide without the sulphate, 17.3 per cent of the
phosphoric acid was soluble in i per cent citric acid, while the
compound formed when ferrous sulphate was present showed
19.0 per cent citric acid solubility.

It is improbable therefore that ferrous sulphate can be of
any benefit as a retentive agent for phosphoric acid, especially
when there is sufficient ferric hydroxide present.

46

CONCLUSIONS

Summarizing briefly, the writer thinks that the following
conclusions are supported by the foregoing work.

I . Lime increases considerably the solubility of precipitated
ferric phosphate in i per cent citric acid solution. The nature
of the calcium phosphate formed depends on the relative amount
of lime added. The reaction takes place with moderate rapidity.
The solubility of dufrenite is increased by the addition of lime,
though not to a very marked degree.

II. Reverted phosphate of iron, formed when phosphoric
acid is absorbed by ferric hydroxide, is not identical with normal
ferric phosphate. . It is much less soluble in i per cent citric
acid. The addition of lime to reverted phosphate of lime in-
creases its solubility in i per cent citric acid. The solubility in

hydrochloric acid is increased only to a slight extent. The

solubility in neutral ammonium citrate solution is not increased.
Reverted phosphate of iron appears to be a highly basic compound,

III. Lime applied in moderate amounts to the soils used,
increased the solubility of the phosphoric acid, in i per cent citric

acid, though the increase was small. The solubilitv in — r—

200

hydrochloric acid was affected to a less degree, and in one of the

solis used no action occurred. The latter reagent, as a solvent

for the available phosphoric acid in soils, is of doubtful utilit)^

IV. The phosphoric acid originally present in soils is
much less susceptible to the action of chemical agents than re-
verted phosphate of iron. The latter probably becomes more
resistant with age.

47

V. The presence of calcium carbonate in relatively large
amounts, appears to have no effect on the solubility, in am-
monium citrate, of the compound formed by fixation.

VI. Insoluble humic acid, calcium humate, and insoluble
ammonium humate offer no resistance to the extraction of the
phosphoric acid of superphosphate by water. Soluble ammonium
humate forms with superphosphate solution, an insoluble com-
pound whose composition and properties have not been deter-
mined. The absorptive property of humus is due probably to
the humus compounds soluble in water.

VII. Soluble ammonium humate does not effect solution of
precipitated tri-calcium phosphate. Insoluble humic acid renders
a portion of the phosphoric acid soluble in water.

VIII. Some of the zeolites and similar hydrous double sil-
icates are capable of abstracting phosphoric acid from solution.
The property is not destroyed by strong ignition. The com-
pounds formed are comparatively soluble in i per cent citric acid.

IX. The solubility of tri-calcium phosphate, ferric phos-
phate, apatite, and wavellite, in water and in i per cent citric
acid, was not increased by the addition of ferrous sulphate.
With tri-calcium phosphate, the solubility in i per cent citric acid
was decreased, indicating that the application of ferrous sul-
phate under some circumstances may be detrimental.

X. Ferrous sulphate forms with superphosphate a compound
soluble in i per cent citric acid. In the presence of ferrous sul-
phate, and ferric hydroxide, the compound formed with super-
phosphate, has practically the same solubility in i per cent citric
acid as the compound formed when ferrous sulphate is absent.
The latter possesses therefore no especial value as a retentive
agent for phosphorics acid.

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